Field of the Invention
The present invention is directed to solid NASICON electrolytes in which the zirconium site is doped with a 2+ oxidation state cation. The present invention is also directed to methods of making the solid electrolytes and methods of using the solid electrolytes in batteries and other electrochemical technologies.
Background
The development of high capacity, high power, and low cost electrochemical batteries will help to catalyze the coming energy revolution. For example, inconsistent production of energy could be averaged over several days or weeks with large, inexpensive batteries. And, vehicles could run on electric motors powered by energy dense batteries, ultimately drawing their power from the electric grid.
Lithium-ion batteries are some of the best performing and most prevalent batteries today. Unfortunately, they bring with them a host of problems, most notably a limited supply and the high cost of lithium. See “The Trouble with Lithium 2: Under the Microscope,” Meridian International Research 2008. Current lithium-ion batteries also entail short cycle-life and dangerous overheating scenarios. See J. J. Ciesla, J. Power Sources 18:101-107 (1986). Moreover, conventional lithium-ion batteries suffer drawbacks due to their organic liquid electrolyte, including dissolution of electrodes into the electrolyte and development of a “solid-electrolyte interface” (SEI), which decreases round trip efficiency and greatly shortens cycle life. See Arora, P., et al., J. Electrochemical Soc. 145:3647-3667 (1998); Smart, M. C., et al., J. Electrochemical Soc. 146:3963-3969 (1999); Blyr, A., et al., J. Electrochemical Soc. 145:194-209 (1998); and Y. Shin and A. Manthiram, Chem. of Materials 15:2954-2961 (2003). Further, the breakdown voltage of liquid electrolytes is only 4V, and liquid electrolytes have been shown to out-gas and explode, which limits the operating voltage and temperature of the battery. See M. Na, Solid State Ionics 124: 201-211 (1999); Y. Shin and A. Manthiram, Chem. of Materials 15:2954-2961 (2003); K. Xu, Chem. Rev. 104:4303-4417 (2004); J. J. Ciesla, J. Power Sources 18:101-107 (1986); Wang, Q. S., et al., Electrochemical and Solid State Letters 8:A467-A470 (2005); Hyung, Y. E., et al., J. Power Sources 119:383-387 (2003); and Xiang, H. F., J. Power Sources 173:562-564 (2007).
All-solid-state sodium-ion batteries promise a cheap, safe alternative to current battery chemistries. Solid state ceramic electrolytes show no electrode dissolution or SEI formation, have been shown to be stable beyond 5V (see Hayashi, A., et al., Nature Commun. 3:856 (2012) and Bates, J. B., et al., J. Electrochemical Soc. 142:L149-L151 (1995)), and are safe to use at very high temperatures due to the intrinsic stability of ceramics. However, the room temperature conductivities of ceramic sodium electrolytes are usually several orders of magnitude lower than their organic counterparts. See J. W. Fergus, Solid State Ionics 227:102-112 (2012); E. J. Plichta and W. K. Behl, J. Power Sources 88:192-196 (2000); and N. D. Cvjeticanin and S. Mentus, Phys. Chem. Chem. Phys. 1:5157-5161 (1999).
If solid-state sodium-ion batteries are to be competitive, they must have high performance at room temperature and thus, be high conductivity solid electrolytes. Superionic NASICON (Na+ Superionic Conductor), Na3Zr2Si2PO12, is one of the most promising and widely studied solid electrolytes. However, the conventional formulation of NASICON provides insufficient performance at room temperature, requiring the use of the higher temperature rhombohedral phase.
Originally developed by Hong and Goodenough, Na3Zr2Si2PO12 exhibits fast sodium ion mobility through a three-dimensional solid network. See H. Y.-P. Hong, Mater. Res. Bull. 11:173-182 (1976) and Goodenough, J., et al., Mater. Res. Bull. 11:203-220 (1976). The mobility of base NASICON is affected by substitutional doping, and understanding this effect provides a meaningful route to enhance the sodium conductivity. In Boilot, J. P., et al., J. Solid State Chem. 73:160-171 (1988), it was demonstrated that adjusting the silicon and phosphorus levels markedly altered the conductivity of Na1+xZr2SixP3−xO12, and could be maximized at x=2 as a result of sodium interactions and sodium oxygen interatomic distance. In contrast, doping the zirconium site in Na3Zr2Si2PO12 is not as readily understood in regard to how it modifies NASICON structure and conductivity. Improvement in the conductivity of Na3Zr2Si2PO12 was achieved by substituting Zr with 4 mol % yttrium, but the higher total conductivity was explained by enhancing microstructure and greater density rather than a structural or chemical change. See Fuentes, R., et al., Solid State Ionics 140:173-179 (2001) and Fuentes, R., et al., J. European Ceramic Soc. 21:737-743 (2001).
Several studies have been published investigating trends in doping effects. See Miyajima, Y., et al., Solid State Ionics 124:201-211 (1999); M. Na, Solid State Ionics 124:201-211 (1999); Saito, Y., et al., Solid State Ionics 58:327-331 (1992); Takahashi, T., et al., Solid State Ionics 1:163-175 (1980); A. Feltz and S. Barth, Solid State Ionics 9:817-821 (1983); and Miyajima, Y., et al., Solid State Ionics 84:61-64 (1996). It has been demonstrated that ionic conductivity increases with transition metal radius doped at the octahedral zirconium site. See Saito, Y., et al., Solid State Ionics 58:327-331 (1992) and Miyajima, Y., et al., Solid State Ionics 84:61-64 (1996). However, the evidence for this trend has been drawn from doping the silicon-free and much less conductive NaZrP3O12 (NZP) compositional end member. Unlike NaZrP3O12 which is stable in the rhombohedral structure at room temperature, Na3Zr2Si2PO12 shows a transition to a low temperature monoclinic phase around 175° C. See Alpen, U. V. et al., Materials Research Bulletin 14:1317-1322 (1979); Feist, T., et al., Thermochemica Acta 106:57-61 (106); and Bukun, N. G., Ionics 2:63-68 (1996).
Following this trend to the extreme, Miyajima showed that the largest ion soluble in NZP is dysprosium, after which maximum solid solubility drops to near dilute doping levels. See Miyajima, Y., et al., Solid State Ionics 84:61-64 (1996).
No such trend has been discovered in supertonic NASICON. Solid solubility in NASICON may not be as tolerant to large radii, as suggested by Takahashi's observation of low yttrium solubility in NASICON despite the solubility of dysprosium—having a large ionic radius—in NZP. See Takahashi, T., et al., Solid State Ionics 1:163-167 (1980).
Solid NASICON electrolytes prepared using Al2O3, Fe2O3, Sb2O3, Yb2O3, and Dy2O3 as dopants were disclosed in U.S. Patent Application No. 2015/0249262.
The present invention provides several NASICON type materials where the metal ion is sodium and in which the zirconium site has been doped with a +2 oxidation state cation. Surprisingly, it has been found that aliovalent dopants can increase the conductivity of the solid electrolyte.